It is known in the art to improve certain chemical reaction kinetics by increasing the pressure and temperature at which the reaction is conducted. One method of achieving the desired conditions has been to provide a deep well hydraulic column or gravity pressure reactor vessel such as disclosed in U.S. Pat. Nos. 3,853,759 and 4,792,408.
Such gravity pressure reactors often comprise a lengthy passageway or column, positioned inside a bore which has been excavated in the strata of the earth. It has been known to house the reactor in an outer casing which is then grouted in place in the bore. This outer casing serves to protect the reactor column from being affected by the surrounding strata, which often contains mineral rich fluids which may be detrimental to the reaction and/or corrosive to the reactor column. Further, the grouting material serves to prevent intermixing of fluids from different stratum and protect the casing from these fluids, as well as acting to prevent heat losses from the apparatus.
It is also known in the art to grout containment casings in place by first introducing grout material into the interior of the casings. A plug is then placed over the grout material and water is pumped behind the plug, such that the water pressure causes the plug to travel downwardly through the casing, pushing the grout material ahead. At the bottom of the casing, the grout material is forced through an open valve and encounters the earth strata and is forced to travel back upwardly along the exterior of the casing, that is, between the casing and the strata, toward the upper end of the casing where the grout material was introduced. The material is then allowed to set, and thereby provides support for the casing.
As disclosed in U.S. Pat. No. 4,792,408, supercritical water temperatures and pressures are often attained in gravity pressure reactors. During routine operations, the temperatures in the reactors often cycle by as much as 900.degree. F. Temperature fluctuations in this range induce compression stresses in the casings as they are caused to expand and yet are restrained by the grouting and the surrounding strata. For example, it has been found that for every 12.degree. F. rise in casing temperature, a unit stress of 2,430 lbs/in.sup.2 is induced into the casing. If the casing has a maximum working stress of 100,000lbs/in.sup.2, the maximum service temperature would be 512.degree. F.; however, modern gravity pressure reactors have peak service temperatures of 900.degree. F. Thus, the compression stresses are likely to exceed the yield strength of the casing material, rendering the casing subject to tension failure when it is cooled for servicing.
Previous attempts at compensating for thermally induced compression stresses in gravity pressure reactor casings have included pre-stressing the casings. This procedure involves anchoring the bottom of the casing, such as by concrete, and exerting a pulling force at the top of the casing, such as by hoists or jacks. As an example, a 400,000 pound casing may require 300,000 pounds of pretension. Thus, a total of 700,000 pounds of lifting force would be required at the top of the casing. Then as such a pre-stressed casing cycles between the divergent temperature ranges, relative degrees of pre-tension would be relaxed and the casing would remain within working stress ranges. However, in addition to requiring such a large amount of lifting force, a further drawback to such a procedure is that the maximum pre-tension is achieved at the top of the reactor casing. The greater need for pre-tensioning is at the bottom of the casing where the temperature fluctuations are the greatest. An additional complication with pre-tensioned casings is that the surrounding strata itself will often expand under the influence of thermal fluctuations. When this occurs, additional tension may be directed to the casing.
It has not proven to be a solution to increase the thickness of the casings. Very deep reactors of over 12,000 feet have limitations in collapse pressures arising from the hydraulic head produced by unset grout on the outside of the casing. If the walls are too thick, then the temperatures on either side of the casing will be great enough to cause internal stresses in the casing walls.
Another drawback to making the casing walls thicker is that the thicker the wall, the more rigid the casing tends to be. This is a problem in that bore holes in the earth strata are not uniform, and often have slight lateral fluctuations. A slight fluctuation extended over miles in length causes a great overall deviation. If the reactor casings are not tolerant of these fluctuations, then they will not be securely positionable inside the bore.
Thus, a need exists for a gravity pressure reactor vessel casing which is flexible enough to compensate for variations in bore dimensions, and which is further capable of compensating for thermal expansion.